1. Field of the Invention
The present invention relates to an exhaust diffuser for an axial-flow turbine.
2. Description of the Prior Art
In an axial-flow turbine, working fluid that has come out of the last stage of rotating blades is exhausted through an exhaust diffuser. FIG. 12 is a schematic axial cross section of a part of an axial-flow turbine 1 showing the positional relationship between the rotating blades and the exhaust diffuser. The axial-flow turbine 1 is a gas turbine having a turbine casing 10 where a plurality of blade rows/stages, each of which consists of stationary vanes 11 and rotating blades 12, are installed. The rotating blades 12 are installed to a rotor 13, and the rear end of the rotor 13 is supported by a bearing (journal bearing) 15 in a bearing housing 14. The bearing housing 14 is supported by a plurality of struts 20 which cross the flow of the working fluid in radial direction so that the bearing housing 14 is concentric with the center of the turbine casing 10. The term “strut” herein means a structure including a supporting member and a fairing that covers the supporting member to reduce the resistance against the fluid.
The arrow F in FIG. 12 shows the flow of the working fluid. The working fluid flowing out of the rotating blades 12 in the last stage is exhausted through an exhaust diffuser 16. The exhaust diffuser 16 consists of a hub-side tube 17 and a tip-side tube 18 concentrically placed with each other to form an annular flow passageway in between. The hub-side tube 17 has a cylindrical form while the tip-side tube 18 has a truncated conical shape whose diameter becomes larger toward the downstream side. Therefore, the cross-sectional area of the flow passageway in the exhaust diffuser 16 increases from the upstream side to the downstream side, making a so-called conical diffuser. The struts 20 maintain the shape of the annular flow passageway by keeping the gap between the hub-side tube 17 and the tip-side tube 18 and supporting these tubes in the turbine casing 10.
Examples of the struts or the fairings covering them can be seen in the Japanese Patent Application Laid-Open No. 2002-5096 or in the Japanese Patent Application Published No. H6-3145.
A gas turbine 1 as shown in FIG. 12 needs to be provided with a large and long exhaust diffuser 16. In order to construct such a large and long diffuser 16, the struts are installed at both the upstream and the downstream sides. The struts 20 shown in FIG. 12 are disposed at the upstream side and will be referred to as “front struts” hereinafter. The struts disposed at the downstream side (outside of the range of FIG. 12) will be referred to as “rear struts” hereinafter.
Although struts are indispensable elements to compose the exhaust diffuser 16, they inevitably cause loss in the exhaust flow. This will be explained by referring to FIG. 13 and the following.
FIGS. 13 through 15 are schematic cross sections of the exhaust diffuser 16. FIG. 13 shows the view of the rear struts 21 seen from the front struts 20. FIG. 14 is a cross section shown at the location of the front struts 20, and FIG. 15 is a cross section shown at the location of the rear struts 21. In these figures, double lines indicate the front struts 20 and thick solid lines indicate the rear struts 21. The front struts 20 are tangential struts and there are six of them. The rear struts 21 are radial struts and there are three of them.
When the exhaust steam flows through the exhaust diffuser 16, a wake occurs behind each of the front struts 20. Various effects are encountered, depending on the relationship between the wake and the rear struts 21.
Like as FIG. 15, FIGS. 16 through 19 are also schematic cross sections of the exhaust diffuser 16 shown at the location of the rear struts 21. These figures show the results of simulations of the exhaust flow separation at various phase angles (i.e. offset angles in the axial-flow direction) of the rear struts 21 relative to the front struts 20.
FIG. 16 shows the result of a simulation performed with the rear struts 21 placed at a phase angle of zero degree relative to the front struts 20. Separation of the exhaust flow occurs on the side surfaces of the rear struts 21, thus deteriorating the exhaust performance.
FIG. 17 shows the result of a simulation performed with the rear struts 21 placed at a phase angle of minus 7.5 degrees relative to the front struts 20. Separation of the exhaust flow occurs on the inner surface of the tip-side tube 18, resulting in deterioration in the exhaust performance. The degree of the deterioration in the exhaust performance here is far greater than in the case of the simulation performed at zero-degree phase angle.
FIG. 18 shows the result of a simulation performed with the rear struts 21 placed at a phase angle of 225 degrees relative to the front struts 20. Separation of the exhaust flow occurs on the outer surface of the hub-side tube 17, thus deteriorating the exhaust performance. The degree of the deterioration in the exhaust performance here is approximately the same as in the case of the simulation performed at a phase angle of zero degree.
Though not visualized in a figure, around a phase angle of 350 degrees, there is a point where the exhaust performance gets extremely deteriorated.
FIG. 19 shows the result of a simulation performed with the rear struts 21 placed at a phase angle of 135 degrees relative to the front struts 20. Separation of the exhaust flow scarcely occurs and the degree of deterioration in the exhaust performance is minor.